Various types of pavers are available for applying material such as asphalt, concrete or the like, to a surface. A common concern in the operation of all pavers is the control of the grade and slope of the material laid on the surface. The grade is the height of the material laid with respect to a grade reference which may be a previously laid material or a string line which is sensed by a grade sensor. The slope is the side-to-side inclination of the material laid down with respect to gravity.
Towed screed pavers typically include a tractor having actuators, which may be hydraulic rams, on either side of the tractor which adjust tow points in a vertical direction. Support or tow arms having first ends are coupled to the tractor at the tow points and the second ends are coupled to either side of a screed. The screed is towed behind the tractor while a supply of material to be laid is fed ahead of the screed. The screed rests on and forms the material as the screed is towed forward and leaves a layer of material behind at the grade and slope of the screed. The tow point elevations are controlled to adjust the attack angle of the screed which ultimately determines the grade and slope of the applied material with respect to the grade reference.
Prior art automatic slope controllers for pavers control screed slope by operating the actuators to control the relative elevation of the two tow points. A change in the relative elevation of the two tow points eventually creates a change in the slope of the screed. However, the actual slope of the screed may not be exactly equal to the commanded slope due to various factors, such as manufacturing and assembly tolerances and the like. Therefore, a gravity or other slope sensor has been provided on the screed. Such controllers have, however, been found to be unstable in operation due to system response delay. This delay is present because the screed cannot instantaneously change slope in response to a change in relative elevation of the two tow points.
In order to overcome the foregoing problem, it has been proposed to use a slope sensor supported by the tow arms at a point forward of the screed. Such a sensor is disclosed in Burgin, U.S. Pat. No. 3,782,844. However, in such prior controllers having a slope sensor on the tow arms, the slope signal does not represent the actual slope of the screed but the slope (i.e., the difference in elevation) of the tow arms at the points of support of the sensor. As a result, an error is introduced into the controller which reduces positioning accuracy.